1. Field of the Invention
The present invention relates to a spot-weldable lightweight composite material, and particularly relates to a spot-weldable lightweight composite material having excellent bending rigidity.
2. Description of the Prior Art
There has been proposed an automobile having a lightweight body in order to decrease the fuel consumption. In order to decrease the weight of car body, the car body is made into a smaller size, the front engine-front driving system mechanism is used, the thickness of automotive steel sheet is decreased by using high tensile steel sheet, and further a weight-saving material is used. However, even when high tensile steel sheet is used in portions, for example, ceiling and the like, which are required to have rigidity, the sheet thickness cannot be decreased, and these portions have not yet been made into light weight.
The use of composite material of a so-called laminate steel sheet, which consists of two steel sheets and an intermediate resin layer having a thickness larger than the sum of the thicknesses of the steel sheets, can decrease greatly the body weight without decreasing rigidity. The composite material has been disclosed by General Motors Corp. and National Steel Co. in a Meeting of SAE, held in Detroit in U.S.A. in February, 1980, and has created a sensation. However, the resin, that is, an organic polymer material, to be used in the intermediate layer of laminated steel sheet, is a good insulating material, and therefore the laminated steel sheet has a serious drawback that the sheet cannot be subjected to a spot welding, which is an indispensable treatment in the material for automobile, although the use of a large amount of the sheets as the material can be expected. In order to obviate this drawback, fastening by bolt, adhesion by adhesive and other various particular methods have been attempted at the use of the laminate steel sheet. However, the drawback has not yet been completely solved.
Composite materials consisting of two metal sheets and a resin held between the metal sheets have hitherto been known not only as a weight-saving material but also as a composite type vibration-damping steel sheet, which absorbs vibration energy by utilizing the viscoelasticity of the resin. A method of giving the spot weldability to the composite type vibration-damping steel sheet is disclosed in Japanese Patent Laid-Open Application No. 128,687/78, wherein a resin is mixed with metal powders, such as iron powders and the like.
However, the laminated steel sheet to be used for weight saving has a very thick intermediate layer dissimilarly to vibration-damping steel sheet, and therefore even when metal powder is merely contained in the intermediate layer, it is very difficult to give spot weldability to the laminated steel sheet to be used for weight saving. The inventors have proposed in Japanese Patent Application No. 32,407/81 a method of compounding graphite powder into an intermediate layer in order that a vibration-damping steel sheet can be spot welded. However, spot weldability was not able to be given to a laminated steel sheet by the method described in the above described Japanese Patent Application No. 32,407/81 due to the difference between the vibration-damping steel sheet and the laminated steel sheet in the thickness of intermediate layer.